Metallurgical Failure Analysis and Fracture Patterns
Metallic materials, which are used in a variety of products, commonly break due to their surrounding environment. The cause of breakage can only be determined after observing the material and accurately analysing the fractured surface.
In automotive, aerospace, and other industries where safety is important, the materials used is the basic component ensuring the quality of the product.
In this section, metallurgical failure analysis methods as well as features of fracture patterns and fractured surfaces are described. This section also introduces examples of using a 4K Digital Microscope to solve problems in metallurgical failure analysis faced by conventional microscopes.
- Causes of Breakage Revealed by Metal Fracture Surfaces
- Types of Fractography
- Fracture Patterns of Metallic Materials
- Problems in Metallurgical Failure Analysis and Their Solutions
- Making Metallurgical Failure Analysis More Advanced and More Efficient
Causes of Breakage Revealed by Metal Fracture Surfaces
Metallic materials such as steel, copper, and aluminium alloy are used in a wide range of products, from home appliances and toys to infrastructure facilities and equipment.
New high-performance materials are being constantly studied, primarily in the automotive and aerospace industries. The studies have become necessary due to requirements to reduce the size and weight of products while also increasing their rigidity and performance. In fields such as automobiles, airplanes, ships, railroad cars, and manned spaceships, breakage of metallic materials may put human lives at risk, so strict material selection and safety design based on strength calculation are required.
When selecting metallic materials, various material tests regarding the stress of such materials are conducted.
Typical test methods are as follows.
- Mechanical tests:
- Tension test, bending test, compression test, shearing test, creep test, wear test, etc.
- Hardness tests:
- Indentation hardness test, dynamic hardness test
- Chemical tests:
- Corrosion test
Through material tests and fractography, causes of breakage and material properties are analysed to select or improve materials.
Types of Fractography
Fractography investigates how metallic materials have fractured (fractured surface pattern or fracture shape) through structure observation to estimate primary causes by considering various aspects such as materials, manufacturing methods, shapes, and use conditions. Among several methods for observing the structure of a fractured surface, the following are the main fractographic methods for metallic materials.
Macroscopic observation
Macroscopic observation is a method of analysis using, for example, the naked eye, low-magnification loupes, and stereoscopic microscopes. This observation can be conducted easily on the spot where a fracture has occurred and is used to roughly distinguish causes based on the type of the fracture, the presence of beach marks, and so on. However, macroscopic observation alone is not enough to investigate in detail how a fracture has occurred.
Microscopic observation
Microscopic observation investigates microscopic features by observing the structure of a fractured surface using optical microscopes and scanning electron microscopes (SEMs). With this observation, you can investigate fracture shapes in detail by capturing various features of a fractured surface, such as dimples and striped patterns.
Fracture Patterns of Metallic Materials
Fracture patterns are classified into ductile, brittle, fatigue, and environmental fractures. For each pattern, macroscopic and microscopic observation is performed to investigate the cause of the breakage. The overview of each fracture pattern and the summary of its fractured surface are described below.
Ductile fracture
A ductile fracture is a fracture pattern observed in many metallic materials and shows large deformation, such as stretching and necking, up to the occurrence of the fracture. Through structure observation, you can see the following features and detailed fracture patterns.
- Features of fractured surfaces
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- Macroscopic observation:
- Shear lip, dull greyish white
- Microscopic observation:
- Microvoids, isometric dimples (tensile fracture), stretched dimples (shear fracture)
Brittle fracture:
A brittle fracture is a fracture pattern in which cracking has rapidly spread while showing little plastic deformation. As cracking proceeds, no plastic deformation occurs around the fractured surface. In general, brittle fractures occur in many steel materials under normal use. In many cases, the fractured surface consists of quasi-cleavage fracture surfaces, which are observed on large heat-treated steel and general structural steel located in extremely cold environments.
- Features of fractured surfaces
-
- Macroscopic observation:
- Silver-white shining reflection, chevron pattern (rapid transgranular fracture), radially spreading cracks
- Microscopic observation:
- Quasi-cleavage fracture surface, river pattern, granular fracture, complex fracture
Fatigue fracture
A fatigue fracture is a fracture pattern in which cracking has gradually proceeded under repeated load. It is said that more than 70% of fracture patterns on mechanical structures fall under this pattern.
The appearance of the fractured material does not show stretching or necking, which is similar to brittle fractures, but significant plastic deformation is revealed under microscopic observation.
The fractured surface is generally smooth, compared to the surfaces of other fracture patterns, and beach marks are observed as a macroscopic feature. From the appearance of these beach marks, you can tell where the fracture started and in which direction the cracking proceeded.
As a microscopic feature, a striped pattern called striation is typically observed. This striped pattern is vertical to the direction in which the crack proceeded and is said to occur easily on aluminium alloys and copper alloys while not occurring easily on ferrous alloys.
- Features of fractured surfaces
-
- Macroscopic observation:
- Beach mark, ratchet mark (several points of stress concentration), fish eye (start point of fracture), radially spreading cracks
- Microscopic observation:
- Striation (corresponding to the stress cycle), secondary cracks, rub marks or fractures with no features
Environmental fracture
An environmental fracture is a fracture pattern that occurs with cracking developing under a corrosive environment. Therefore, this fracture may also occur even under an extremely small external stress.
Typical environmental fractures are hydrogen embrittlement and stress corrosion cracking.
- Hydrogen embrittlement
- Hydrogen embrittlement is also called a delayed fracture and is commonly observed on steel materials. This phenomenon is caused by hydrogen entering into materials. This entry is typically observed in material manufacturing processes, such as welding and electroplating, and in corrosion reactions in the usage environment.
- Stress corrosion cracking
- This cracking often occurs in accidents in which austenite stainless steel is the material. In particular, transgranular fractures often occur in usage environments with C1- ions. On the other hand, grain boundary fractures occur in materials other than stainless steel such as pure copper, brass, and aluminium alloys.
- Features of fractured surfaces
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- Hydrogen embrittlement
-
- Macroscopic analysis
- Silver-white shining reflection
- Microscopic analysis
- Granular fracture, hair mark
- Stress corrosion cracking
-
- Macroscopic analysis
- Partial reflection, rusting/discolouration
- Microscopic analysis
- Granular fracture, feathering pattern
- High-temperature fracture
-
- Macroscopic analysis
- —
- Microscopic analysis
- Granular fracture, dimple, sinkage
Problems in Metallurgical Failure Analysis and Their Solutions
As described above, by observing fractured surfaces not only through macroscopic observation but also through microscopic observation, you can investigate fracture patterns in more detail to understand the causes and conditions of fractures. In many cases, microscopes and scanning electron microscopes (SEMs) are used for microscopic observation, but they have several problems in metallurgical failure analysis.
You can solve these problems and capture various fracture patterns with high definition to ensure more reliable analysis by using our 4K Digital Microscope.
This section introduces examples of solving problems in metallurgical failure analysis using KEYENCE's VHX Series ultra-high-definition 4K Digital Microscope.
Removing glare on a metal fracture surface
Conventional problems with microscopes
Irregular reflection on a metal fracture surface can generate glare, which sometimes makes it difficult to observe cracking. Unclear observation may cause inaccurate analysis due to overlooked cracking.
With the VHX Series 4K Digital Microscope
The glare removal function can suppress unnecessary reflection so that you can clearly capture even minute cracking on a metal fracture surface.
Fully focusing on the entire target even for uneven metal fracture surfaces
Conventional problems with microscopes
Fractured surfaces on metallic materials are generally three dimensional. To observe each feature of the many irregularities on a fractured surface, you need to adjust the focus repeatedly, resulting in analysis requiring a lot of time. Another problem is that comprehensive observation based on an entire target image is not possible.
With the VHX Series 4K Digital Microscope
The real-time depth composition function makes it possible to bring an entire metal fracture surface into focus. This function not only reduces the time required for adjusting the focus repeatedly, but also enables you to observe and evaluate many composite features that exist on a fractured surface.
Analysing details irrespective of angle and shadow
Conventional problems with microscopes
Irregularities on a metal fracture surface not only make it difficult to adjust the focus, but also change the shadowing depending on the angle. As a result, it is difficult and takes a lot of time to determine the lighting conditions. In some cases, it is also difficult to explain fracture patterns only with image data captured under a single lighting condition.
With the VHX Series 4K Digital Microscope
Using the multi-lighting function that automatically captures omnidirectional lighting data at just the push of a button, you can select the image that is most suitable for structure observation.
Even after you have selected or exported captured images, image data with each lighting condition is still saved on your PC and can be recalled.
Clear observation even for minute shapes of subtle patterns
Conventional problems with microscopes
Some metal fracture surfaces may have subtle fracture patterns. In such cases, fractography requires a lot of time and the contrast may be too low to carry out observation properly.
With the VHX Series 4K Digital Microscope
The Optical Shadow Effect Mode, a new observation method that combines a specifically designed high-resolution HR lens, a 4K CMOS image sensor, and lighting, analyses the contrast in an image captured with varied illumination.
This method makes it possible to clearly observe subtle and minute irregularities on a metal fracture surface. Irregularity information can also be displayed in different colours by composing an Optical Shadow Effect Mode image with colour information.
Making Metallurgical Failure Analysis More Advanced and More Efficient
As explained above, the VHX Series high-definition 4K Digital Microscope makes it possible to easily view metal fracture surfaces that could not be observed well with conventional microscopes and SEMs.
Because the VHX Series can greatly reduce the time spent on metallurgical failure analysis, you can increase the speed of the quality improvement cycle and R&D and achieve much higher work efficiency compared to when using a conventional microscope or a SEM. Also, the VHX Series enables you to recall previous image data, so you can select and improve materials using past trends and comparisons.
Equipped with many other advanced functions, the VHX Series can be a powerful partner for more efficient fractography and structure observation, which are required to become a leader in the field of R&D. For additional product info or inquiries, click the buttons below.
